The term “wet friction material” is well understood by those skilled in the art. These materials typically use a porous substrate comprising fibers such as wet laid cellulose and possible other additives. Rings of this substrate are typically mounted on each side of a ring-shaped metal support disk. The substrate is partially impregnated with a curable resin dissolved in a solvent with the resin most widely used being a phenol type. In a standard transmission or clutch, these members are mounted in an interleaved relation with bare metal reaction disks or plates, and the assembly can be moved in and out of engagement, for example, to provide a shifting or braking function in a vehicle. These assemblies run in lubricating liquids to provide, among other things, a cooling function.
In wet friction materials, it is desirable to have as high of a coefficient of friction as possible. Other desirable properties include durability and high energy capacity.
It is taught in the literature that softer, generally referring to lower modulus, binder/substrate systems, have higher friction coefficients. The compliancy of the system, the ability to conform to the reaction plate surface, maximizes surface contact during clutch engagement. This helps dissipate heat as well, reducing coning, hot-spotting, while improving durability.
Phenolic resin systems, in particular phenol formaldehyde—based resoles, are the predominate saturating resins used in the friction industry. Unmodified, these resins are brittle and have relatively high modulus. To lower modulus, improve fatigue resistance, and in general improve friction properties, these resin are modified, usually with elastomeric compounds such as nitrile or gum rubber. In addition, they may be made more “linear”, hence more flexible, by using substituted phenols, such as cresol, in place of phenol, and/or acetaldehyde, propionaldehye, benzaldehyde, etc., in place of formaldehyde. However, even extensive modification does not fully overcome the inherent brittle nature of phenolic systems. There are both chemical and practical limitations, such as solution viscosity, which govern the possible extent of chemical modification of these systems.